Extension of the blood half-life of glyceryl trinitrate. Inhibition of glutathione organic nitrate ester reductase activity in the rat and guinea pig

Xanthine oxidoreductase catalyses the reduction of nitrates and nitrite to nitric oxide under hypoxic conditions

Xanthine oxidoreductase (XOR) catalyses the reduction of the therapeutic organic nitrate, nitroglycerin (glyceryl trinitrate, GTN), as well as inorganic nitrate and nitrite, to nitric oxide (NO) under hypoxic conditions in the presence of NADH. Generation of nitric oxide is not detectable under normoxic conditions and is inhibited by the molybdenum site-specific inhibitors, oxypurinol and (-)BOF 4272. These enzymic reactions provide a mechanism for generation of NO under hypoxic conditions where nitric oxide synthase does not function, suggesting a vasodilatory role in ischaemia.

Organic nitrate reductase: reassessment of its subcellular localization and tissue distribution and its relationship to the glutathione transferases

1. Using a specific and sensitive GLC method for the determination of glyceryl trinitrate (GTN), its subcellular and tissue distribution were reassessed. Liver was the most active tissue, but activity was also detected in the heart, kidney and gut. In all tissues activity was localized in the soluble fraction. The activity of soluble glutathione S-transferase followed the same pattern, liver exhibiting the highest and the heart the lowest activity. 2. Pretreatment with phenobarbitone and 3-methylcholanthrene stimulated both the glutathione S-transferase and organic nitrate reductase activities. 3. Glutathione S-transferase activity was competitively inhibited by GTN. 4. A comparison of the plasma and hepatic metabolism of GTN revealed higher drug affinity for the hepatic enzyme.

Role of the liver in the disposition of intravenous nitroglycerin in the rat

Previous studies have indicated that the liver is the main site of nitroglycerin (NTG) elimination when the drug is systematically infused. To examine this hypothesis, we measured the apparent systemic clearance (Cls) of nitroglycerin in anesthesized rats receiving a constant intravenous infusion at a dose of 100 micrograms per kg per min. Animals were divided into shunt and sham groups; the former had undergone a portal vein ligation 10 days prior to the study, while the latter was subjected to a sham operation. On the study day, half of the animals of each group also received probenecid at 200 mg/kg, i.v., a drug previously reported to inhibit organic nitrate ester reductase (ONER) activity in rat liver. Arterial NTG samples were obtained at 41, 43 and 45 min of infusion in all four experimental groups; Cls was 439 +/- 32 ml per kg per min (mean +/- S.E.) in sham, 460 +/- 44 in sham and probenecid, 477 +/- 39 in shunt, and 461 +/- 34 in shunt and probenecid animals. During NTG infusion, hepatic blood flow (measured with a constant infusion of indocyanine green) was decreased markedly in shunted rats as was liver/body weight, indicating hepatic atrophy. The specific activity of hepatic ONER was similar in all four groups. In spite of marked differences in hepatic blood flow and hepatic mass, the Cls was similar in all four groups. The liver does not appear to be a major site for the elimination of systemic nitroglycerin as hitherto assumed.

Pharmacokinetics of nitroglycerin after parenteral and oral dosing in the rat

The pharmacokinetics of nitroglycerin was characterized in detail using venous plasma after different intravenous bolus doses (0.15-2.48 mg/kg), intra-arterial infusion (8.2 micrograms/min over 5 h), and oral doses (7-100 mg/kg). Venous plasma clearance was found to be approximately 650 mL/kg and was independent of the intravenous or intra-arterial dose. This confirmed earlier reports that the venous plasma clearance of nitroglycerin in rats exceeded the value of normal cardiac output. A terminal half-life of approximately 15 min was observed after high intravenous bolus doses of nitroglycerin. This slow disappearance phase was likely rate limited by redistribution of drug back into the plasma. The bioavailability of oral nitroglycerin (F) showed an apparent Michaelis-Menten dependency on dose. F was less than 5% at doses less than 20 mg/kg, but increased to a plateau of approximately 20% from 50-100 mg/kg. First-pass metabolism of nitroglycerin is thus apparently controlled by at least two systems (sites or enzymes). Coadministration of mannitol hexanitrate, a potential competitive inhibitor of first-pass metabolism, did not increase F.

Application of high-performance liquid chromatography with synchronized accumulating radioisotope detector to analysis of glyceryl trinitrate and its metabolites in rat plasma

A new, sensitive, and specific high-performance liquid chromatographic method for the quantitative analysis of [14C]glyceryl trinitrate and its four metabolites in plasma is described. The drugs are extracted from 0.05 ml of plasma with methanol and analyzed by high-performance liquid chromatography using a synchronized accumulating radioisotope detector. The limit of detection is 0.2 ng per injection. The within-day coefficient of variation is 5.9% at a concentration of 27.0 ng per ml of plasma. The method was applied to single-dose pharmacokinetics of glyceryl trinitrate in rat.

Potentiation of the cardiovascular effects of nitroglycerin by N-acetylcysteine

The biochemical basis of the mechanism of vasodilatation by nitroglycerin (NTG) has not been previously investigated in man. However, evidence from in vitro studies suggests that NTG induces activation of guanylate cyclase via a series of enzymatic reactions that are modulated by the availability of sulfhydryl groups. Cysteine appears to be particularly effective in potentiating guanylate cyclase activation by NTG. To determine whether hemodynamic responsiveness to NTG in man might be modulated by sulfhydryl availability, concentration-response curves for effects of intravenously infused NTG on mean arterial pressure (MAP) and mean pulmonary capillary wedge pressure (PCW) were obtained in 10 patients undergoing cardiac catheterization for investigation of chest pain. NTG infusion was repeated 10 min after the intravenous infusion of 100 mg/kg of the cysteine source N-acetylcysteine (NAC). NAC induced no significant hemodynamic effect, but after NAC infusion there was a significant reduction both in the NTG infusion rate associated with a 10% fall from control values in MAP (25.8 +/- 8.3 to 9.3 +/- 2.7 micrograms/min; p less than .01) and in the infusion rate inducing a 30% reduction in PCW (13.6 +/- 4.6 to 4.2 +/- 1.6 micrograms/min; p less than .02). In a control group of five patients who received no NAC, there was no significant change in responsiveness to NTG between infusions. It is concluded that NAC potentiates the vasodilator effects of NTG in man. This suggests that sulfhydryl availability and/or redox state may be determinants of in vivo responsiveness to NTG.

Animal pharmacology of nitroglycerin

Nitroglycerin, first introduced over a hundred years ago, is now finding wider clinical applications. To a large degree, the renewed interest in the clinical pharmacologic usage of nitroglycerin is due to the availability of new formulations and drug delivery systems. The current review focuses on the physiological and pharmacological actions of nitroglycerin in mammals. Routes of nitroglycerin metabolism, biochemistry and absorption are discussed. The phenomenon of nitroglycerin tolerance is illustrated and related to specific quantitative alterations occurring at the cellular level. The cardiovascular effects of nitroglycerin are discussed in terms of its effects on coronary flow, the myocardium itself, and on the peripheral vasculature. The early speculation of Murrell (3) that nitroglycerin "would probably prove of service in the treatment of angina pectoris ... and other vascular disorders ..." has now been realized.